Metallic halide electric discharge lamps



Feb. 8, 1966 G. H. RElLlNG METALLIC HALIDE ELECTRIC DISCHARGE LAMPS Filed Jan. 23, 1961 /fs A tllror-h ey.

United States Patent() Filed Sian. 23, 1961, Ser. No. 34,068 Claims. (Cl. 313-25) 'I'he present invention relates to high eiciency gaseous electric discharge lamps. More particularly, this invention relates to such lamps which emit light from a gaseous arc of vaporized mercury and one or more vaporized metallic halides.

In the lighting arts, the mercury yarc lamp has achieved a place of commercial acceptance because of its long life and reasonably high etiiciency. One great disadvan-tage of the mercury arc lamp, however, has been the bluish green light which it characteristically emits. This light is not pleasing to the human eye and does not illuminate as Well as White light because of poor color rendition. Additionally, a substantial amount of energy isV lost without an attendant production of visible light from mercury arc lamps due to the emission of ultraviolet light. This and other energy losses place a limit of useful eiciency of approximately 55 lumens per arc-watt eiciency upon presently commercially available mercury arc lamps.

Accordingly, one object of lthe present invention is to provide arc lamps having improved eiciencies and more pleasing spectral emission with a minimum of loss by ultraviolet radiation.

Another object is to provide arc lamps having high efciency Without an attendant loss in desirable radiation characteristics.

Still another object is to provide arc lamps having a pleasing near-White emission Without the lincorporation of auxiliary light-emitting means.

Yet another object is to provide arc lamps having high efficiencies and which may emit any one of a variety of preselected colors.

Another object of the present invention is to provide` improved methods and criteria for operating arc lamps so as to obtain therefrom the most desirable combination o-f emission and eliiciency characteristics.

A further object of this invention is to provide improved -arc lamps suitable `for general illuminating purposes which are highly efficient, produce pleasing White or near- -White emission and are readily, reproducibly and inexpensively manufactured as a commercial product.

Briey, in accord with one aspect of the present invention, I provide an arc lamp including an hermetically sealed bulb containing a pair of non-liquid arc-electrodes, enough readily ionizable gas therein to initiate a gaseous discharge therein with the application of operating potentials, a quantity of mercury Which is completely vaporized during operation to produce a hot mercury arc discharge and a quantity of at least one vaporizable metallic halide. In -further accord with my invention the lamp envelope is juxtaposed With respect to the electrodes so that the coldest portion thereof is always, during operation, hot enough to insure that an etective amount of the metallic halide becomes and remains vaporized. In accord withV another feature of the invention ltwo or more of these halides may be present within the envelope. In accord with one embodiment of the invention, I include Within the envelope a quantity of excess halogen to prevent any possible depletion of the halide.

The novel features believed characteristic of the present invention are set forth in the appended claims. Further objects and vadvantages of the invention, both as to organization and mode of operation, are set forth in the 3,234,421 Patented Feb. 8, 1966 ICC following detailed description thereof and the accompanying drawing in which:

FIG. 1 is a vertical view, with parts broken away, of an arc lamp constructed in accord with .the present invention,

FIG. 2 is a graph illustrating the improvement in eficiency of one lamp constructed in accord with the invention over a conventional mercury -arc lamp after 1000 hours of operation, and

FIG. 3 illustrates the comparable improvement in efciency of other typical lamps in accord With the invention.

Referring now to FIG. 1 of the drawing, an arc lamp constructed in accord with one embodiment of the present invention may include a iirst, outer light transmissive vitreous envelope 1 having therein a second, inner lighttnansmissive envelope 2. Envelope 2 contains therein a pair of oppositely disposed non-liquid arc-electrodes 3 and 4 and an ignitor electrode 5 located closely adjacent to one arc-electrode lSi. A charge 6 comprising a quantity of mercury together with a quantity of a metallic halide is located in envelope 2 at the lower end thereof. Envelope 2 is suspended Within envelope 1 by a suspension assembly 7 Which comprises a pair of vertical suspension rods 8 supporting therebetween a plurality of saddle straps 9 which bind the constricted ends -10 of envelope 2. A lead-in lwire 1'1 connected to arc electrode 4 passes through the upper end 1t) of envelope ,2 and a lead-wire C12, is connected to arc-electrode 3, passes through the opposite end 10 thereof. lA third lead-in Wire 13, connected to starting electrode S, passes through the same end of inner envelope 2 as lead-in Wire 12. Lead-in Wire 11 is connected through one of supporting rods 8 to an input lead 14, while lead-in Wire 12 is connected to a second input lead 1S. Starting'electrode 5 is connected through lead I13 and a voltage-dropping resistance 16 toinput lead 14. Input leads |14 and 15 are sealed in an hermetic seal through the pressed end portion .17 of a ret-entrant portion 18 of outer envelope y1 and are electrically connected to respective terminal portions of base cap 19 thereof.

The outer envelope of the lamp or" FIG. 1 may be conveniently constructed of a suitable glass, and is utilized to maintain the operative portions of the lamp in a protective atmosphere to prevent oxidation at high temperature. The inner envelope thereof is conveniently constructed of quartz, or other light-transmissive substances capable of withstanding inner wall temperatures Y of the order of at least 600 C.-1200 C. Arc-electrodes 3 and 4 are non-liquid refractory metallic members capable of withstanding the high temperatures of arc footpoints. Preferably, electrodes 3 and 4 are composed of tungsten and are activated to high electron emissive characteristics by an associated activator such as metallic thorium. One convenient structure, illustrated, includes a rod of tungsten and a sliver of thorium with a coil of tungsten wrapped thereabout. Alternatively, if a halide of a sufficiently lowV Work function metal is present in the arc, the electrodes may be initially unactivated. lgnitor electrode 5 may conveniently be of tungsten. The inner support members comprising support assembly '7 may conveniently be constructed of nickel, stainless steel or other materials conventionally utilized in such applications. The space within inner envelope 2 may include a quantity of non-reactive ionizable gas such as a noble gas suflicient to cause the establishment of a gaseous arc discharge therein upon the application of operating voltages to arc-electrodes 3 and 4 and starter electrode 5. This filling is conventional in mercury arc lamps and may conveniently be argon gas at a pressure of approximately 15 millimeters of mercury.

Charge 6 contains a sufficient amount of mercury so that when the mercury is entirely vaporized during the operation of the arc lamp, it provides a pressure in excess of one atmosphere and normally from in excess of one to approximately 15 atmospheres of mercury within the enclosure of envelope 2. ,This results in the characteristic radiation spectrum of mercury. Charge.6 also contains an amount of a metallic halide salt suicient so that when the mercury is entirely vaporized and the coldest portion of the interior Wall of envelope 2 is at a temperature in excess of approximately 600 C., an effective amount of the metallic halide is vaporized and remains in the vapor state. The amount of halide in the vapor state whichV is eiective is that which is suicient to constitute within envelope 2 a partial pressure of approximately 10-3 to l03 mm. of Hg pressure of vaporized halide, although in order to obtain maximum eiiciency I have found it desirable that the partial pressure of the halide be from l to 200 mm. of mercury pressure. These amounts vary, depending upon the vapor pressure of the halide chosen.V

Some metallic halides which may be utilized, in accord with the present invention, are the iodides of lithium, sodium, cesium, calcium, cadmium, barium, mercury, gallium,' indium, thallium,Y germanium, tin, thorium, selenium, tellurium and zinc. The most eiiicient metals fall in the alkali metal group, the alkaline earth metal group and Group Illb of the periodic table of the elements. While these iodides are preferred, the bromides and chlorides of these metals as well may be used with improvements in efciency and in color over conventional mercury arc lamps. All of the 4foregoing iodide additives v resulted in the attainment of white or near-white light and/or high eliciency. Thus, for example, the addition of thallous iodide, although not resulting in greatly improved color, does result in the attainment of eiiiciencies as high as 90 lumens/ watt., In addition to the foregoing, other iodides, bromides and chlorides may be utilized to produce light having a'preselected characteristic Wave length other than white light when such is desirable. Due to the intense vchemical attack on quartz and other vitreous substances,V iiuorides are not suitable as the metallic halide utilized in the practice of Ythe invention particularly with quartz envelopes.

The preferred metallic halides utilized for highest efciency in accord with the present invention are those including metals having strong resonance radiation spectra or strong recombinationradiation spectra in theV visible spectrum. Resonance radiation is radiation emitted by atransition of an excited atom from the lowest excited state from which such a' transition is permitted by spectroscopic rules to the ground or unexcited state. For the production of white or near-white light of highest eiiiciency at least one metal present in the lamp should have its resonance line in the yellow or red .portions of hte visible spectrum in order to combine with and complement the mercury spectrum. Y Recombination radiation results from the recombination of a free electron with a metallic ion to form a neutral atom. Such recombination radiation exists as a continuum and for certain elements has the appearance of near-white light.

In accord with .another feature of my invention, I have found that although increased eiciencies Yand a more useful color of light, as compared with the mercury arc, are obtained by the use of single metallic halides in conjunction with mercury, an unexpectedly much greater increase in eiciency and improved color characteristics as compared with mercury arc lamps is obtained when certain combinations of iodides are used. Thus, when sodium iodide is utilized in conjunction with either thallium or thallous iodide the results are outstanding. For example, I have obtained eiiciencies of 100 lumens per watt and the production of a high purity near-white light using sodium iodide and thallium with the mercury filling of an arc lamp. Similarly, I have achieved similar emission spectra and efficiencies in excess of 100 lumens per watt using mercury, sodium iodide and thallous iodide. Other combinations may be made with advantage. Thus, for example, other metallic iodides have been added to lamps containing mercury, thallous iodide and sodium iodide in arc lamps. These further additions ill out the lamp spectrum, improving color rendition. To date the best results in color rendition, with eiciencies as high as 8O lumens per watt have been obtained with an arc lamp having a filling (in addition to a gaseous filling for starting purposes) of mercury, sodium iodide, thallous iodide andindium iodide.

As has been set forth hereinbefore, the quantity of added solid metallic halide need only be an amount suficient to result, at the lamp operating temperatures, in ya partial pressure of approximately l03 toY103 mm. of Hg pressure of vaporized halide in the inner lamp envelope.

' Generally,rths is achieved by placing an excess of the halide, as for example 50 mg. within the enevelope, since the vapor pressures of the useful metallic halides are so low that the problem is one of getting enough halide in the vapor state and this is controlled by controlling the envelope inner wall temperature. 'When, however, two or more metallic halides are utilized in order to achieve a desired balanced light output, it becomes a matter of controlling the amount of halide present in the solid or liquidstate so that, considering the desired operating temperature and the vapor pressures of the different constituents, the desired partial pressure of each is achieved. Thus, although no'control need generally be exercised over the halide having the lowest vapor pressure, the con# stituent or constituents having higher vapor pressures are often pla-ced in the enevelope in small quantities, it being contemplated Vthat all of these constituents will become vaporized at the operating temperature necessary to vaporize an elective amount of the constituent having the low vapor pressure. One example of this balancing is shown in a 400 watt lamp with an inner envelope volume of 25 cubic centimeters and electrode spacing of 7.5 centimeters having, as the filling, Vthe following:

Mm. Argonv -..Y l5 Mercury 8G IThallous'iodide z 5 Sodium iodide 40 Iodine ,Y 50

In this mixturethallous iodide has a higher vapor pressure than sodium iodide and it is desired to vaporize al1 of the former. Y

The maximum eiciency of approximately 100 lumens per watt for this lamp was obtained at an inner envelope outer wall temperature of 850 C. (inner wall temperature of approximately 900 C.). At this temperature the par# tial pressure of thallous iodide was approximately 100 mm. Hg and the partial pressure of sodium iodide was approximately 10 mm. Hg. The partial pressure of mercury was approximately 4 atmospheres.

, In the'operation of arc lamps` in accord with the present` invention, it is of the greatest importance thatrthe mercury present be in a relatively small quantity so that Y all ofthe mercury is vaporized. This is because it is essential that the entire inner bulb wall and all exposed l inner surfaces within envelope 2 of the lamp of FIG. l be raised to a temperature in excess or" 600 C. and preferably from 750 C.-1200 C. This is necessary in order to insure that the temperature of the coldest portion of the wall is su'iciently high enough so that'a suicient amount of metallic halide present within the envelope is vaporized to obtain the improvements achieved in accord with the invention. If there istoo much mercury in the envelope so that all the mercury is not evaporated, as, for example, ifa molten pool'of mercury were utilized as one electrode for the lampthe temperature of at least one portion of the bulb Wall, namely in the vicinity of the pool electrode would, in the absence of prohibitively high pressures of mercury vapor much greater than atmospheres, not rise above the boiling point of mercury for that pressure.

The maximum temperature for the mercury pool under these conditions and, consequently, for the adjacent portions of the bulb Wall would be the equilibrium temperature for mercury for that pressure and, up to the pressures at which lamps in accord with the present invention operate, would be only slightly in excess of 355 C., the atmospheric pressure boiling point of mercury. These tempratures are insuflicient to cause the evaporation of a sufficient amount of halide or to sustain a sufficient amount of halide in the vapor phase to cause proper operation of lamps in accord with the present invention. Under these conditions, any halide which becomes vaporized as by contact with the arc footpoints would be vaporized only transiently and then would seek out the coldest spot in the bulb wall and condense thereupon, leaving only the mercury vapor Aas a substantial continuous light-emitting source, with the color correction' amounting to only a iiickering component. Thus, although it lhas been proposed by the prior'art to add certain metallic halides to a mercury arc lamp in order to add red constituents to the blue of the mercury arc emission and thus produce White light, the proposed lamps always incorporated a mercury pool electrode and operated at low temperatures and low pressures so that no continuous color correction, and much more important, no substantial increase in efficiency Were realizable therefrom. This is because the addition of metallic halides to mercury lamps under the conditions attempted in the prior art results in substantially no continuous emission of light by the halide or its constituents but rather, result only in a short lived lllckering intermittent modification of the mercury arc emission.

To the extent that such color correction did occur in prior art devices it could only last for a matter of a few hours. Thus, so long as the mercury is present in liquid form, the additive tends to clean up, reacting chemically with the mercury, becoming dissolved or admixed with the mercury, and becoming permanently deposited upon the cold Walls adjacent the pool. This process is further accelerated by the action of the arc in preferentially seeking out a clean mercury surface to conserve energy and maintain a minimum arc voltage. This action of the arc further keeps any additive oating on the surface of the mercury from contacting the arc by virtue of the pressure blast created at the surface of the mercury pool'by explosively vaporizing mercury.

In accord with the present invention, however, metallic halides are utilized in an arc discharge lamp that is operated under conditions which permit the halide to become and remain volatilized so as to constitute an essential and important continuously light-emitting constituent of the lamp which contributes in large part to the improved color and increased efliciency obtainable therefrom.

The lamps ofthe present invention may be characterized as intermediate pressure arc lamps. The pressure of mercury vapor (the main partial pressure) is adjusted to be in excess of one to approximately 15 atmospheres of mercury vapor by including in envelope 2 a quantity of mercury that, when entirely vaporized, yields the desired pressure at the temperature conditions found in normal lamp operation. lf the minimum temperature of the envelope is reduced so that the mercury pressure falls to one atmosphere, then the temperature will likewise be insucient to vaporize, and maintain vaporized, a suicient amount of metallic halide. lf, on the other hand, the mercury pressure exceeds approximately l5 atmospheres and the pressure of the halide exceeds approximately one atmosphere, then the temperatures of the arc and the envelope Wall would be such as to adversely affect the bulb Wall. Adverse edects may in- .6. clude 'devi'trication," clouding yand iiracturiiig.Y Addi# tionall at highy pressures a broadening and linally an intense inversion of the resonance spectral lines of the line spectrum of the metal atom` of the dissociated halide occurs, with a consequent loss in efliciency. Inversion is characterized by a darkening of the center of the spectral line and pressure broadening of the line. While moderate inversion of the spectral'lines may be tolerated, the intense inversions which occur at pressures of halide in excess of approximately one atmosphere in lamps of this type are undesirable. This intense inversion is caused by self absorbtion of the emitted resonance radiation by the metal atoms of the dissociated metallic halide not in the vicinity ofthe center of the arc, and some subsequent non-radiative deexcitation of the absorbing atoms. The inversion is greater as the density of unexcited metallic atoms between the arc and the bulb wall increases.

The ultimate parameters upon which operation of the invention depend vare the vapor density within the envelope, the arc temperature, and the internal envelope wall temperature. The tirst of these parameters, the total vapor density is maintained at a value of the order of 1018 to 102 atoms of vapor per cubic centimeter and is important to insure that the arc temperature may be maintained Within a proper range; to prevent ionizing electrons from migrating to the envelope Wall; and to prevent (should the vapor density become too high) broadening of the spectrum and line inversion.

The second of these parameters, the arc temperature is maintained at a temperature of approximately 3000 C.6000 C. (at the center of the arc column) to insure that the electrons in the arc have sufcient energy 'to energize the metal atoms of the halide to resonance em'isjsion, rather than exciting them to undesirable emission utside the visible spectrum.

insure that an effective amount of metallic halide is vaporized and maintained in the vapor state.`

The upper inner bulb wall temperature of 12.00 C.

is not a limit upon the mode of operation of the lamps but merely represents the temperature at which quartz (the material generally used to fabricate envelope 2) becomes unduly soft. transmissive envelope, such as the high density alumina disclosed and claimed in the copending application S.N.v

743,829 tiled June 23, 1958, of R. L. Coble and assigned to the .present assignee is used, a higher upper operating temperature may be used.

When, however, quartz inner envelopes are utilized the operating Wall temperature should not exceed approximately 1200 C. Likewise since iodine is the least active of Athe halogens and there is a possibility of halogen attack of quartz, when inner envelope 2 is of quartz 'the iodides are preferred as the halides Within the envelope.y

The internal Wall temperatureV is controlled by juxtaposing the envelope Wall suiciently close to the arc so l that the heat of the arc keeps the wall temperature at the required value. Additionally,`the lamp is 'operated' at voltages and currents sufficient to supply enough' power to the arc to secure the required envelope wall`V temperature. One means for this operation is the voltage means indicated generally at 20 and including a suitable auto transformer Z1 and an alternating voltage sourcey 22. 400 Watt lamps have been operated from a General Electric auto transformer, Cat. No. 86Gl4, in this fashion.

As an additional control feature means may be provided within the outer envelope l and external to inner envelope 2 to prevent the loss of heat from the wall of envelope 2. Thesemeans may include evacuation ofthe space to a hard vacuum of, for example, l micron or less,

lling the space with a low pressure of a heavy molecular weight gas such as xenon or packing the volume surround- If a higher temperature light-` ing envelope 2 with a material which selectively transmits visible and blocks infrared radiation, such as quartz wool. While packing with quartz wool would be expected to decrease light output, it has unexpectedly increased lamp efiiciency to values as high as l20lumens per watt.

Another means to increase lam-p efficiency comprises the placing of heat reflecting shields about the cathodeadjacent end regions of inner envelope 1 to maintain the Vcathode-adjacent regions of the inner wall hot. Normally in conventional arc lamps the addition of such shields decreases total light output. Quite unexpectedly, however, the addition of such heat shields to the lamps of the invention so increases the eiciency of the lamps that the total light output thereof is increased by the addition of the shields. Such increases have amounted to as much as 10%. Such heat shields may comprise thin reilecting layers indicated as 23 in FIG. l of the drawing on the ends of envelope 2 (shown only at the upper end in the drawing), or a -closely spaced exterior reflecting shield which encloses these regions of the inner envelope end.

IFIG. 2 illustrates the lumens per arc-watt efficiency, as a function of arc-watt input, for a mercury lamp (curve A) and a typical lamp in accord with the present invention utilizing mercury, sodium iodide, and thallium after the lamp had been in operation for 1000 hours (curve B). As may readily be seen from the drawing, at approximately 500 arc-watts input, the lamp in accord with the present invention (curve B) exhibits a lumen efficiency of approximately 100 lumens per arc-watt, Whereas the conventional mercury lamp exhibits an efficiency of less Vthan 60 lumens per arc-watt.

In FIG. 3 of the drawing, it may seem that a lamp having a filling of mercury, thallous iodide and sodium iodide has an efficiency characteristic very similar to curve B of FIG. 2. Although other halide additives do not equal the 100 lumens per arc-watt efficiency of the mercury, sodium iodide-thallous iodide lamp, nevertheless the addition of mercury iodide, barium iodide, or calcium iodide all result in the achievement of higher efficiencies than mercury alone, together with the added advantage that all of these lamps produce white or near-white light, whereas the conventional mercury are emits greenish-blue light. Additionally, as shown, FIG. 3, a lamphaving a filling of Hg, TlI, NaI and InI vhad an eficiency of approximately 8O lumens per watt. This lamp exhibited an even more pleasing white light than the lamp having a filling of Hg, TlI andNaI. v Y

In both FIGS. 2 and 3 the curves represent data taken by testing identical lamps (other than the filling) under identical circumstances in the Vsame integrating sphere. This made possible a comparison of the efficiencies of the material constituting the arc independent of`other inuences.

In the operation of lamps in accord with the present invention, a Voltage is first applied to arc-electrodes 3 and 4 and starter'electro'de 5. Because of its close proximity,

starter electrode 5 serves as one electrode for an immediate glow discharge which takes place between Vit and arc electrode 3, the ionized gas comprising the discharge being the noble starting gas contained therein. This discharge'causes the`heating of electrode 3 and creation of a sufiicient number of nobleV gas ions to cause the main discharge gap between arc-electrodes 3 and 4 to break down. The heat of the noble gasv arc'vaporizes the mercury in charge 6 and, when a sufiicient number of mercury ions have been created by the thermal action of the noble gas arc, the lamp glows blue with the characteristic mercury discharge. As is Well known to the art other means may be utilized to obtain a mercury arc. As the mercury discharge continues to build up in intensity and further heats the interior walls of interior envelope 2, the vapor pressure of the' metallic iodide increases. .As the inner wall of envelope 2 approaches approximately 600 C., a sufiicient amount of halide is vaporize'd and remainsin the vapor state to cause the necessary operat- 8 ing partiall pressure thereof to exist throughout envelope 2. When this occurs, a suiiicient number of molecules of vaporized halide are dissociated by the intense temperature of the arc center (approximately 3000 C. or higher), energized by the arc and emit their characteristic line spectra, changing the total light emission from the arc to a pleasing, near-white or white of intense brilliance.

Spectroscopie observations of the light emitted by lamps in accord with the present invention indicate that the radiation which is present therein, in addition to the characteristic mercury line spectrum, is the line spectra of the metal of the halide or halides utilized. Accordingly, it is believed that, in the operation of the lamps of the present invention, the metallic halide particles are thermally dissociated andY excited by collisionsv with electrons and mercury atoms (stable or metastable) or ions giving them suicient energy to later radiate their characteristic line spectra. It is due to this mode of operation that the lamps of the present invention derive their high efhciencies.

Consi-der the case of the mercury arc lamp. Since the minimum excitation potential of mercury is approximately 4.5 ev., an electron must have that amount ot energy to excite a mercury atom to emit resonance radiation. At the arc temperature (approximately 3000 C.-6000 C.) relatively few electrons have this energy.

Consider next `an arc or" a metal halide only. The excitation potential of the metal of the halide may be as low as 2.1 ev. for sodium. Due to the temperature stability of the metallic halides, however, it is not possible, at the moderate temperatures and pressures of the halides utilized in lamps of the type described, to supply to the arc sufficient power at reasonable voltages and currents to produce high brightness light. This maybe accomplished only if the metallic halide is present at very high temperatures and pressures. Such high halide pressure results in reversal of the resonance line and a spectral shift. While this may be tolerable in some lamps, it is undesirable in lamps of the type herein considered. Additionally, any metallic atoms produced by dissociation at moderate pressure of halide vapor are not readily excited because, with a low density halide arc alone, the electrons which should excite, migrate instead to the envelope walls and are lost.

Consider now the lamps of the invention. The mercury readily vaporizes and is ionized to form a high temperature arc which is sufliciently hot to dissociate a significant portion of the vaporized metallic iodide molecules. The metal atoms of the halide are then readily excited by electrons, atoms and ions in the arc plasma. Since the elements may be chosen to have alow minimum excitation potential such as sodium (2.1 ev.), thallium (3.3 ev.) or indium (3.0 ev.) they are readily excited by electrons having energies insuicient to excite mercury atoms (4.5 ev.). Additionally, relatively few exciting electrons escape from the arc to the envelope wall, since they are retained within the arc'plasma by elastic Vcollisions with the high density of mercury atoms.

From the foregoing, it is evident that because of the presence of mercury vapor within the envelope, a higher gas density is present than would be present for the same temperature if the iodide were the only vapor contained within the lamp. This makes it possible to achieve the benefit of the radiation from the excited metallic atom of the halide without unreasonably raising the temperature of the envelope and without raising the pressure of halide vapor within the envelope to the extent that would be necessary in order to achieve reasonable brightness and eliciency from an arc of the metallic halide alone. This is a great adavntage, since if the halide vapor had to be operated at the temperatures which would be necessary if the mercury were not present, a greater amount of infrared radiation lossfrom the envelope wall and from the arc would occur, thus lowering the efficiency of the lamp. Additionally, the relatively low temperature mode of operationand relatively low iodide density reduces corrosion n the lamp envelope and electrodes, since the chemical attack of halides upon these materials is very low at low temperature and low pressures, but is highly destructive at the temperatures and densities necessary to maintain an arc in metallic halide vapor alone. For reasons of this nature, in the preferred embodiment of the invention, even though three or more halides may be utilized, the total halide pressure during lamp operation does not exceed one atmosphere.

In accord with another feature of the invention I have found it desirable to operate the lamps of the invention in an atmosphere of excess halogen vapor. I have found that, although lamps operated without an excess of free halogen are reasonably free of discoloration near the electrodes, when free halogen vapor is present even less darkening of the envelope wall in the vicinity of the electrodes occurs. This is believed due to the fact that, during operation, some of the halogen present due to a dissociation of the metallic halide reacts chemically with the cathode material leaving an excess of metallic vapor present. This vapor is believed to condense out when the lamp is cooled. With an excess of free halogen, there is always sutiicient halogen to prevent precipitation or deposition of a free metal on the bulb wall Whether this metal cornes from the halide filling or from the cathode itself. In practice a quantity of free halogen the same as the halogen of the halide sufficient to yield, at operating temperature, a partial pressure of l to 100 mm. of mercury of 'excess halogen over the stoichiometric amount of the halide is ideal for this purpose.

In one lamp constructed in accord with the present invention, the internal diameter of inner envelope 2, FIG. 1 was approximately 1.5 centimeters and the length between the arc-electrodes was 7 centimeters. Voltage applied between the electrodes was 200 volts RMS. at 2.2 amperes. argon and had 200 milligrams of mercury and milli-v grams of thallium and 5 milligrams of sodium iodide. Additionally, 5 millimeters of mercury partial pressure of .iodine gas was added in excess of the iodide. This device operated for 1300 hours at an elhciency of from 70-80 lumens per watt at an operating level of approximately 400 watts before failing. When it failed, failure was due to the fact that the interior envelope had been operated in air rather than in an atmosphere of a non-reactive gas, as is desirable, and the exterior lead lll from the inner envelope corroded and broke. The interior of the device was not, however, incapable of operation.

VA further advantage of lamps in accord with the invention is a reduction of loss of eiciency by ultra-violet radiation.

The reduction thereof is believed due in part to the formation of thin, perhaps monomolecular, layer of metallic halide upon the inner surface of the walls of envelope 2 which absorbs the ultraviolet radiation. The energy contained therein helps to heat the envelope wall. This thin layer is also responsible for another beneficial etect. The use of vapors of salts of such active metals as sodium was thought to be inadvisable because of an expected attack by the active metals upon the envelope` wall. Due to the presence of a thin layer of the halide upon the envelope interior, no such attack has beenevident.

While the invention has been set forth herein with respect to certain embodiments thereof, many modifications and changes will readily occur to those skilled in the art. Accordingly, I intend, by the following claims, to include all such modications and changes as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent by the United States is:

1. A gaseous electric discharge lamp comprising: an hermetically sealed light-transmissive envelope; a pair of The bulb was filled with l5 millimeters of non-liquid metallic arc-electrodes extending within said envelope; a quantity of mercury within said envelope sufficient upon complete evaporation thereof during operation of the lamp to yield a mercury pressure of only approximately 1 15 atmospheres therein providing during operation a high temperature arc discharge; and a quantity of metallic iodide selected from the group consisting of the iodides of lithium, sodium, cesium, calcium, cadmium, barium, gallium, indium, thallium, mercury and zinc within said envelope suliicient to yield under operating conditions a partial pressure of approximately l0-3 to 200 mm. Hg of vaporized iodide therein, said iodide under operating conditions being dissociated by and within the column of said are and being substantially undissociated in the volume between said arc column and the interior envelope wall, the walls of said envelope being spaced with respect to said electrodes so that during operation the coldest portion of the interior wall of said envelope is maintained at a temperature in excess of 600 C. and no higher than 1200 C.

2. A gaseous electric discharge lamp comprising: an hermetically sealed light-transmissive envelope; a pair of non-liquid metallic arc-electrodes extending within said i envelope; a quantity of mercury within said envelope sucient-upon complete vaporization thereof during operation of the lamp to yield a pressure of mercury of only approximately 1-15 atmospheres therein; a quantity of at least one metallic iodide selected from the group consist* ing of the iodides of lithium, sodium, cesium, calcium, cadium, barium, gallium, indium, thallium, mercury and zinc within said envelope suiiicient to yield under operating conditions a partial pressure of approximately 10-3 to 200 mm. Hg of said vaporized iodidetherein, said iodide under operating conditions being dissociated by and within the column of said arc and being substantiall undissociated in the volume between said arc column and the interior envelope Wall; and a quantity of free iodine within said envelope sufficient to prevent depletion of the iodine content of said iodide during normal lamp life, the Walls of said envelope being spaced with respect to said electrodes so that the coldest portion of the interior Wall of said envelope is maintained at a temperature in excess of 600 C. and no higher than 1200 C.

3. A gaseous electric discharge lamp comprising: an hermetically sealed light-transmissive envelope; a pair of non-liquid metallic arc-electrodes extending within said envelope; a quantity of mercury within said envelope suicient upon complete vaporization thereof during operation of the lamp to yield a pressure of mercury of Vonly approximately ,1 15 atmospheres therein providing during operation a high temperature arc discharge; a quantity of at least one metallic iodide selected from the group consisting of the iodides of lithium, sodium, cesium, calcium, barium, gallium, indium, thallium, mercury and zinc within said envelope sutlicient to yield under operating conditions a partial pressure of approximately 10-3 to 200 mm. Hg of said vaporized iodide therein, and a quantity of free iodine within said envelope suiiicient to yield under operating conditions a partial pressure of iodine vapor of approximately l-200 mm. of mercury in the lamp with the iodide molecule being dissociated by the arc discharge and with the metal component of the dissociated molecule being excited to resonance radiation by the said arc discharge to change the emission spectra and increase the luminous eiiiciency of light emission rom said arc; said iodide existing in the dissociated state substantially only within the column of said a-rc, the walls of said envelope being spaced with-respect to said electrodes so that the coldest portion of the interior wall of said envelope is maintained duringoperation at a ternperature in excess of 600 C. and no higher than l200 C.

4. A gaseous electric discharge lamp comprising: an hermetically sealed light-transmissive envelope; a pair of non-liquid metallic arc-electrodes extending within said envelope sutiicient to establish a glow discharge therein upon the application of an operating voltage between said arc-electrodes; a quantity of mercury within said envelope sucient upon complete vaporization thereof during operation of the lamp to yield a pressure of mercury of only approximately l-l atmospheres Within said envelope to provide during operation a high temperature arc discharge; and a quantity of at least one metallic halide other than a fluoride selected from the group consisting of the halides of lithium, sodium, cesium, calcium, barium, gallium, indium, thallium, mercury and Zinc within said envelope suticient to yield, under operating conditions a partial pressure of approximately *3 t-o 200 mm. Hg of said vaporized halide Within said envelope With the halide molecule being dissociated by the arc discharge and with the metallic component of the dissociated molecule being excited to resonance radiation Within said high temperature arc discharge to change the emission spectra and increase the luminous efficiency of light emission from said arc, said halide existing in the dissociated state substantially only Within the column of said arc, the walls of said envelope being spaced with respect to said electrodes so that during operation the coldest portion of the interior Wall of said envelope is maintained at a temperature in excess of 600 C. and no higher than 1200 C.

5. A gaseous electric discharge lamp comprising: an hermetically sealed light-transmissive envelope; a pair of non-liquid metallic arc-electrodes extending Within said envelope; a quantity of mercury within said envelope suicient upon complete vaporization thereof during operation to yield a pressure of only approximately 1-15 atmospheres therein; respective quantities of sodium iodide and thallous iodide within said envelope each suflicient to yield under operating conditions independent partial pressures of approximately l to 200 mm. of mercury of each iodide therein, said iodide under operating conditions being dissociated by and Within the column of said arc and being substantially undissociated in the volume between said arc column andthe interior envelope Wall, the Walls of said envelope being spaced With respect to saidb electrodes so that during operation the coldest portion of the interior wall of said envelope is maintained at a temperature in excess of 600 C. and no higher than 1200 C. 6. A gaseous electric discharge lamp comprising: an hermetically sealed light-transmissive envelope; a pair of non-liquid metallic arcfelectrodes extending Within said envelope; a quantity of mercury within said envelope sufficient upon complete vaporization thereof during operation of the lamp to yield a pressure of mercury of only approximately 1-15 atmospheres therein to provide during operation Ya high temperature arc discharge; and a quantity of at leastone metallic halide other than a iiuoride selected from the group consisting of the halides of lithium, sodium, cesium, calcium, cadmium, barium, gallium, indium, thallium, mercury and zinc within said envelope suiiicient to yield under operating conditions a partial pressure of approximately 10-3 to 200 mm. of Hg of said vaporized halide in the lamp with the halide molecule being dissociated during operation by the high temperature arc discharge and with the metal component of the dissociated molecule being excited to resonance radiation within said arc discharge to change the emission from said arc; said halide existing in the dissociated state substantially only within the column of said arc, the walls of said envelope being spaced with respect to said electrodes so that during operation of the lamp the coldest portion of the interior wall of said envelope is maintained at a temperature in excess of 750 C. to 1200 C. "7. A gaseous electric discharge lamp comprising: an

hermetically sealed light-transmissive envelope; a pair.

of non-liquid metallic arc-electrodes extending Within said envelope; 'a quantity of mercury Within said envelope sufficient upon complete vaporization thereof during operation of the lamp to yield a pressure or" mercury of only 1-15 atmospheres therein to provide during operation l a vhigh temperature arc discharge; a quantity of at least one metallic iodide selected from the group consisting ofthe iodides of lithium, sodium, cesium, calcium, cadmium, barium, gallium, indium, thallium, mercury and zinc within said envelope sufhcient -to yield under operating conditions a partial pressure of approximately l0-3 to 200 mm. Hg of said vaporized iodide in the lamp with the iodide molecule being dissociated by the arc discharge and with the metal component of the dissociated molecule being excited to resonance radiation Within ysaid arc discharge to change the emission spectra and increase the luminous efficiency of light emission from said arc; said iodide existing in the dissociated state substantially only within the column of said arc, Iand a quantity of free iodine Within said envelope suicient to pre-vent depletion of the iodine content of said iodide during normal lamp life, the walls of said envelope being spaced with respect to said electrodes so that during operation the coldest portion of the interior Wall of said envelope is maintained at a temperature 4in excess of 60 C. and no higher than 1200" C.

8. A gaseous electric discharge device comprising: an hermetically sealed light-transmissive envelope; a pair of non-liquid metallic arc-electrodes extending within said envelope; a quantity of mercury within said envelopeV suiiicient upon complete vaponiz-ation thereof during op eration of the lamp to yield a pressure of mercury of only approximately 1 10 atmospheres therein to provide during operation a high temperature arc discharge; and a quantity of a metallic iodide selected from the group consisting of the iodides of lithium, sodium, cesium, calcium, cadmium, barium, gallium, indium, thallium, mercury and zinc within said envelope suflicient to yield under operating conditions a partial pressure of approximately 1-200 millimeters of mercury of -vaporized iodidefin the lamp with the iodide molecule being dissociated by the arc discharge and =with the metal component of the dissociated molecule being excited to resonance radiation Within said arc discharge to change the emission spectra and increase the luminous eiliciency of the light emission from said arc, said iodide existing in the dissociated-state substantially only within the column of said arc, the Walls of said envelope being spaced with respect to said electrodes so that during operation the coldest portion of the interior wall of said envelope is maintained at a temperature in excess of 600 C. and no higher than 1200 C.

9. A gaseous electric discharge lamp comprising: an hermetrically sealed light-transmissive envelope; la pair of non-liquid metallic arc-electrodes extending Within said envelope; a quantity of mercurywithin said envelope sufficient upon complete vaporization thereof during operation to yield 'a pressure ofV only approximately l-l5 atmospheres therein; respective quantities of sodium iodide and thallous iodide within said envelope sufficient to yield under operating conditions independent partial pressures of approximately 10-3 to 200 mm. Hg of each iodide therein, said iodide under operating conditions being dissociated by and within the column of said arc and being substantially undissociated in the volume between said arc column and the interior envelope Wall,

10. A gaseous electric discharge lamp comprising: 'an 'i hermetically sealed light-transmiss-ive envelope; a pair. of non-liquid metallic arc-electrodes extending within said Q envelope; a qauntity of mercury -Within said envelope suiiicient upon complete lvaporization thereof during operation to yield a pressure of only approximately 1-15 atmospheres therein; respective quantities of sodium iodide and thallous iodide within said envelope suicient to yield under operating conditions independent partial pressures of approximately 10-3 to V200 mm. Hg of each iodide therein, said iodide under operating conditions 13 being dissociated by and within the column ofsaid arc and being substantially undissociated in the volume between said -arc column and the interior envelope wall;

"and a quantity of free iodine Within said envelope suflicient to prevent depletion of the iodine content of said iodides during normal lamp life, the walls of said envelope being spaced with respect to said electrodes so that during operation the coldest portion of the 'interior wall of said envelope is maintained at a temperature in excess of 600 C. and no higher than 1200 C.

11. A gaseous electric discharge lamp comprising: an hermetically sealed light-transmissive envelope; `a pair `of non-liquid metallic arc-electrodes extending within said'envelope; a quantity of mercury within said envelope sufficient upon complete vaptorization thereof during operation to yield a pressure of only approximately =1l5 atmospheres therein; respective quantities of sodium iodide, thallous iodide and indium iodide within said envelope suicient to yield under operating conditions independent partial pressures of approximately 10*3 to 200 mm. Hg of each iodide therein, said iodide under operating conditions being dissociated by and within the column of said arc and being substantially undissociated in the volume between said arc column and the interior envelope wall; the walls of said envelope being spaced with respect to said electrodes so that during operation the coldest portion of the interior Wall of said envelope is maintained at a temperature in excess of 600 C. and no higher than 1200" C.

12. A gaseous electric discharge lamp comprising: an ermetically sealed lighttransmissive envelope; a pair of non-liquid metallic arc-electrodes extending within said envelope; a quantity of mercury within said envelope sufiicient upon complete vaporization during operation to yield a pressure of only approximately 1-15 atmospheres therein; respective quantities of sodium iodide, thallous iodide and indium iodide within said envelope suliicient to yield under operating conditions independent partial pressures of approximately l3 to 200 mm. Hg of each iodide therein, said iodide under operating conditions being dissociated by and within the column of said arc and being substantially undissociated in the volume between said arc column and the interior envelope wall; and a quantity of free iodine lwithin said envelope suicient to prevent depletion of the iodine content of said iodides during normal lamp life, the walls of said envelope being spaced with respect to said electrodes so that during operation the coldest portion of the interior wall of said envelope is maintained at a temperature in excess of 600 C. and no higher than 1200 C.

13. A gaseous electric discharge lamp comprising: an evacuable light-transmissive outer envelope; an hermetically sealed light-transmissive inner envelope supported within said outer envelope; a pair of non-liquid metallic arc-electrodes extending through respective end regions into said inner envelope; a quantity of mercury within said inner envelope suicient upon complete vaporization thereof during operation of the lamp to yield a pressure of only approximately l-l5 atmospheres therein to provide during operation a high temperature anc discharge; a quantity of a metallic iodide selected from the group consisting of the iodides of lithium, sodium, cesium, calcium, cadmium, barium, gallium, indium, thallium, mercury and zinc within said inner envelope sufficient to yield under operating conditions a partial pressure of approximately -3 to 200 mm. Hg of vaporized iodide in the lamp with the iodide molecule being dissociated by the high temperature arc discharge and with the metal component of the dissociated molecule being excited to resonance radiation within said arc discharge to change the emission spectra and increase the luminous efficiency of light emission from said arc; said iodide existing in the dissociated state substantially only within the column of said arc, the walls of said inner envelope being spaced with respect to said electrodes so that the coldest portion of the interior wall of 14 saidinner envelope is .maintained during operationof the lamp at a temperatureV in excess of 600 C. and no higher than 1200 C. 'andheat shield members about the end regions of said inner envelope wall for selectively preventing the loss of heat from said end regions by radiation.

14. The electric discharge lamp of claim 13 wherein the heat reecting shields comprise evaporated metallic layers upon the surfaces of said end regions.

15. A gaseous electric discharge lamp comprising: an evacuable light-transmissive outer envelope, an hermetically sealed light-transmissive inner envelope supported within said outer envelope; a pair of non-liquid metallic arc-electrodes extending within said inner envelope; a quantity of mercury within said inner envelope'suicient upon complete vaporization thereof during operation of the lamp to yield a'pressure of only approximately l-l5 atmospheres-therein to provide during operation a high temperature arc discharge; a quantity of a metallic iodide selected from the group consisting ofthe iodides of lithium, sodium, cesium, calcium, cadmium, barium, gallium, indium, thallium, mercury and zinc within said inner envelope suiicient to yield under operating conditions a partial pressure `of approximately 10-3 to 200 mm. Hg of vaporized iodide in the lamp with the iodide molecule being dissociated by the high temperature arc discharge and with the metal component of the dissociated molecule being excited to resonance radiation within said arc discharge to change the emission spectra and the luminous etiiciency of light emission from said arc; said iodide existing in the dissociated state substantially only within the column of said arc, the Walls of said inner envelope being spaced with respect to said electrodes so that the coldest portion of the interior wall of said inner envelope is maintained during operation at a temperature in excess of 600 C. and no higher than l200 C.; and means within the space between said outer and inner envelope walls for minimizing the loss of heat through said space to maintain said inner envelope wall hot with as low as possible power input to said lamp and thereby increase lamp eiciency.

16. The electric discharge lamp of claim 1S wherein the means for minimizing heat loss comprises a hard Vacuum within said space.

17. The electric discharge lamp of claim 15 wherein the means for minimizing heat loss comprises a low pressure filling of a high molecular weight gas.

18. The electric discharge lamp of claim 15 wherein the means for minimizing heat loss comprises a loosely packed mass of material which is substantially transparent to visible light but blocks infrared radiation.

19. A gaseous electric discharge lamp comprising: an hermetically sealed light-transmissive envelope; a pair of non-liquid metallic arc-electrodes extending within said envelope; a quantity of mercury within said envelope sufficient upon complete vaporization thereof during operation t-o yield a pressure of only approximately l-l5 atmosphercs therein; respective quantities of sodium iodide, thallous iodide and indium iodide within said envelope suticient to yield under operating conditions independent partial pressures of approximately l to 200 mm. of mercury of each iodide therein, said iodide under operating conditions being dissociated by and within the column of said arc and being substantially undissociated in the volume between said arc column and the interior envelope wall; the walls of said envelope being spaced with respect to said electrodes so that during operation the coldest portion of the interior wall of said envelope is maintained at a temperature in excess of 600 C. and no higher than 1200 C.

2t?. A gaseous electric discharge lamp comprising: an hermetically sealed light transmissive envelope; a pair of non-liquid metallic arc-electrodes extending within said envelope; a quantity of mercury within said envelope sufticient upon complete vaporization thereof during operation of the lamp to yield a mercury pressure of only ap- 1 5 proximately 1 to 15 atmospheres therein to provide during operation a high temperature arc discharge; and a quantity of a metallic iodide selected from the group consisting of the iodides of barium, cadmium, calcium, cesium, gallium, indium, lithium, mercury, sodium, thallium, thorium and zinc within said envelope suicient to yield under operating conditions of said arc discharge a partial pressure of approxiately 103 to 200 millimeters of mercury of vaporized iodide in the lamp with -the iodide molecule being dissociated by the high temperature arc discharge and with the metal component of the Idissociated molecule being excited to resonance radiation VWithin said arc discharge to change the emission spectra and increase the luminous eiciency of light emission from said arc; said iodide existing in the dissociated state substantially only within the column of said arc, the walls of said envelope being spaced with respect to said electrodes so that during operation of the lamp the coldest portion of the interior Wall of said envelope is maintained at a temperature in excess of i 16 l 600, C. and no higher than 1200 C. to prevent condensation of said iodide upon the interior wall and maintain a sulicient partial pressure of` volatilized iodide within said envelope to insure sufficient dissociation thereof by the high temperature arc to produce said resonance radiation and increase in luminous efficiency.

References Cited bythe Examiner UNITED STATES PATENTS Pomfrett et al 313-25 GEORGE N, WESTBY, Primary Examiner.

RALPH G. NiLsoN, JOHN W. HUCKERT, Examiners. 

13. A GASEOUS ELECTRIC DISCHARGE LAMP COMPRISING: AN EVACUABLE LIGHT-TRANSMISSIVE OUTER ENVELOPE; AN HERMETICALLY SEALED LIGHT-TRANSMISSIVE INNER ENVELOPE SUPPORTED WITHIN SAID OUTER ENVELOPE; A PAIR OFNON-LIQUIDMETALLIC ARC-ELECTRODES EXTENDING THROUGHRESPECTIVE END REGIONS INTO SAID INNER ENVELOPE; A QUANTITY OF MERCURYWITHINSAID INNER ENVELOPE SUFFICIENT UPON COMPLETE VAPORIZATION THEREOF DURING OPERATION OF THE LAMP TO YIELD A PRESSURE OF ONLY APPROXIMATELY 1-15 DURING OPERATION A HIGH TEMPERATURE ARC DISCHARGE; A QUANTITY OF A METALLIC IODIDE SELECTED FROM THE GROUP CONSISTING OF THEIODIDES OF LITHIUM, SODIUM, CESIUM, CALCIU, CADMIUM, BARIUM, GALLIUM, INDIUM, THALLIUM, MERCURY AND ZINC WITHIN SAID INNER ENVELOPE SUFFICIENT TO YIELD UNDER OPERATING CONDITIONS A PARTIAL PRESSURE OF APPROXIMATELY 10**-3 TO 200 MM. HG OF VAPORIZED IODIDE IN THE LAMP WITH THE IODIDE MOLECULE BEING DISSOCIATED BY THE HIGH TEMPERATURE ARC DISCHARGE AND WITH THE METAL COMPONENT OF THE DISSOCIATED MOLECULE BEING EXCITED TO RESONANCE RADIATION WITHIN SAID ARC DISCHARGE TO CHANGE THE EMISSION SPECTRA AND INCREASE THE LUMINOUS EFFICIENCY OF LIGH EMISSION FROM SAID ARC; SAID IODIDE EXIDTING IN THE DISSOCIATED STATE SUBSTANTIALLY ONLY WITHIN THE COLUMN OF SID ARC, THE WALLS OF SAID INNER ENVELOPE BEING SPACED WITH RESPECT TO SAID ELECTRODES SO THAT THE COLDEST PORTION OF THE INTERIOR WALL OF SAID INNER ENVELOPE IS MAINTAINED URING OPERATION OF THE LAMP AT A TEMPERATURE IN EXCESS OF 600*C. AND NO HIGHER THAN 1200*C. AND HEAT SHIELD MEMBERS ABOUT THE END REGIONS OF SAID INNER ENVELOPE WALL FRO SELECTIVELY PREVENTING THE LOSS OF HEAT FROM SAID END REGIONS BY RADIATION. 